Any of us who had believed, with relief, that the death of Osama bin Laden signaled the end of the terrorist threat to world peace posed by Al Qaeda must now recognize that the threat remains dangerously alive, especially in the Middle East.

“I think we are witnessing a turning point, and it could be one of the worst in all our history,” Elias Khoury, a Lebanese novelist and critic told The New York Times recently. “The West is not there, and we are in the hands of two regional powers, the Saudis and Iranians, each of which is fanatical in its own way. I don’t see how they can reach any entente, any rational solution.”

And Americans must recognize that as long as the Middle East remains politically and militarily volatile, the rest of the world must remain prepared to cope with the threat of terrorism. American support for one side in the Middle East will inevitably invite anger — and possibly acts of terrorism — against the United States — from the other.

When it began more than two years ago, what is now clearly a civil war in Syria no doubt generated considerable American sympathy for the rebels, who were typically portrayed as bravely risking their lives to repudiate the undemocratic leadership of Bashar al Assad, the dictator who openly favored one of the nation’s two major Islamic religious factions (his) over the other.

That portrayal may have been valid at the time, but no longer is it simply a case of freedom lovers fighting to unseat a dictator because the effort to oust Assad has been joined, with brutal vigor, by extremist elements linked to Al Qaeda.

These newcomers have even been fighting other anti-Assad rebels. In fact, across northern Syria over the weekend, there was a rebel uprising against the newcomers, who are associated with the Islamic State of Iraq and Syria (ISIS), now one of the region’s most powerful organizations.

And these newcomers are clearly more interested in forcefully imposing their own brand (Sunni) of Islam on the Syrian people (many of whom are Shiite) than they are in the cause of the original fighters, which was, more simply, to bring justice and fairness to that troubled land.

Now the Syrian civil war has spilled over the border into neighboring Iraq, where it poses a risk to the democratically elected but mostly Shiite government that’s held power since the end of the American-led war that brought the downfall of Saddam Hussein, a Sunni who had often brutalized Iraq’s Shiites.

Last week, after heavy fighting, extremists associated with Al Qaeda seized two important cities in Anbar Province, just west of the Iraqi capital, Baghdad. American troops had fought fiercely — at the cost of 1,300 lives — to subdue insurgencies in these two cities, Fallujah and Ramadi, but now they’re back in enemy hands.

Secretary of State John Kerry, visiting Israel, said Sunday that the United States is ready to help Iraq fight these enemies in every way it can short of a return of American forces.

The Obama administration must worry that the situation in Iraq may presage a similar deterioration of security in Afghanistan once American combat forces are brought home later this year.

“It’s not in America’s interests to have troops in the middle of every conflict in the Middle East, or to be permanently involved in open-ended wars in the Middle East,” Benjamin J. Rhodes, a White House deputy national security adviser, commented Saturday.